osu!

That circle clicking rhythm game that also an aiming game. Based on Osu! Tatakae! Ouendan. Probably the most popular rhythm game along with Guitar Hero and DDR. Spelled with a lowercase o and an exclamation mark at the end.

A: "What do you just play?"

B: "This game I just know. It's called Osu."
A: "IT'S SPELLED osu! U IDIOT TCVBVJGY..."
B: "Haha circle goes brrrrrrrrrrrr..."